The paper Measurement-based Classical Computation (previously called "Exact sampling and entanglement-free resources for measurement-based quantum computation") has been accepted in Physical Review Letters as an Editor's Suggestion.

A new Centre for Doctoral Training in Delivering Quantum Technologies funded by EPSRC will shortly be accepting applications. This provides an excellent opportunity for outstanding students to study a PhD in this research field at UCL. Dan Browne is one of the co-directors of the centre.

In collaboration with Ben Brown (Imperial College) and Earl Campbell (Berlin), Dan Browne and Hussain Anwar have posted a new paper to the pre-print archive arxiv.org developing new error correction procedures for topological codes, and important class of error correcting codes for fault tolerant quantum computation.

Abstract: Qudit toric codes are a natural higher-dimensional generalization of the well-studied qubit toric code. However standard methods for error correction of the qubit toric code are not applicable to them. Novel decoders are needed. In this paper we introduce two renormalization group decoders for qudit codes and analyze their error correction thresholds and efficiency. The first decoder is a generalization of a "hard-decisions" decoder due to Bravyi and Haah [arXiv:1112.3252]. We modify this decoder to overcome a percolation effect which limits its threshold performance for high dimensions. The second decoder is a generalization of a "soft-decisions" decoder due to Poulin and Duclos-Cianci [Phys. Rev. Lett. 104, 050504 (2010)], with a small cell size to optimize the efficiency of implementation in the high dimensional case. In each case, we estimate thresholds for the uncorrelated bit-flip error model and provide a comparative analysis of the performance of both these approaches to error correction of qudit toric codes.

Congratulations to Hussain Anwar. Hussain passed his viva in defense of his PhD thesis entitled "Toward fault-tolerant quantum computation in higher dimensional systems" on October 30th. Hussain has now begun a post-doctoral research associateship at Brunel University.

Popular Summary: A quantum computer exploits the nonclassical aspects of quantum mechanics, but its extreme sensitivity to noise makes fault-tolerant techniques a must for it to operate reliably. A key component in high-threshold fault-tolerance schemes is the preparation of magic states, quantum states in a superposition of classical states, that are required to exploit quantum effects. However, the slightest of experimental imperfections results in the preparation of flawed magic states, unsuitable for immediate use in quantum computers. Fortunately, many copies of flawed magic states can be distilled down to a smaller number of suitable magic states. This process of magic-state distillation, however, forms a bottleneck in implementations of the already proposed fault-tolerant techniques in terms of the impractically high overheads in resources such as memory and running time. Unless overcome, such overhead costs could consign quantum computers to history books as a theoretically possible, but practically infeasible, technology. We propose in this paper that a shift of magic-state distillation from the current qubit paradigm (involving two-level systems) to a qudit one (involving d-level systems) may provide a way to overcome the overhead bottleneck.

Our starting point was curiosity: Can we achieve magic-state distillation in more complex systems? Quantum computing is usually conceived in terms of qubits, but this need not be the case. In fact, qudits can be the basis for a quantum computer, and we designed methods of magic-state distillation for all prime numbers d. To our surprise, by exploiting new insights into number-theoretic properties of qudit computers, some of our protocols offer substantial improvements over their better-known qubit cousins. In particular, five levels appear as the optimal case where our protocol outperforms all previously discovered protocols in all figures of merit. Notably, the potential resource savings in device memory could easily be on the order of a millionfold.

This work opens the door for the development of qudit-based fault-tolerance schemes based on magic-state distillation. These schemes may offer significant advantages over their qubit counterparts and hence bring the development of a scalable quantum computer closer to our grasp.

Dan is local co-organiser of the Second International Workshop on Adiabatic Quantum Computing (AQC 2013), taking place in March 2013 and hosted and organised jointly by University College London and the Quantum Optics, Quantum Information and Quantum Control Group of the Institute of Physics, brings together researchers from different communities to explore this computational paradigm. The goal of the workshop is to initiate a cross-platform dialogue on the implementation challenges that must be overcome to realise useful adiabatic quantum computations in existing or near-term hardware. The workshop will have a special focus on AMO (Atomic, Molecular, and Optical) and solid-state technologies.

Former PhD student in this group, Dr Matty Hoban, has been awarded the Carey-Foster Prize for Outstanding Postgraduate Research Physics in the Atomic Molecular Optical and Positron Physics group at UCL.

A quantum computer would exploit the non-classical aspects of quantummechanics to achieve modes of calculation impossible on conventional"classical" computers. However, constructing a device large enough toperform complex calculations represents a huge challenge for modernphysics. One reason for this is that the delicate quantum statesencoding the information are easily corrupted by the noise induced bythe environment or imperfections in the computer itself. Fortunately,methods of fault tolerant quantum computation have been developedwhich enable reliable quantum computation in the presence of noise. Akey component in many fault tolerance schemes is the preparation ofmagic states, the essential ingredient for non-classical computation.The magic states are prepared by distillation, a process thatexchanges quantity for computational potency.

Quantum computing is usually conceived in terms of qubits (two-levelsystems) but this need not be the case. In fact, systems with anynumber of levels known as qudits can be the basis for a quantumcomputer. Here we prove, for the first time, that magic statedistillation can be achieved in qudits with any prime number oflevels. In particular, we show that for qutrits (three-level systems)magic state distillation works equally well as its qubit counterpart,and in some aspects it is even better. To devise the distillationprocess we employ quantum variants of ideas from classical computerscience, called Reed-Muller codes. The techniques we develop areimportant in their own right, with the quantum Reed-Muller codeshaving useful applications beyond magic state distillation.

This opens the door for the development of qudit-based fault toleranceschemes based on magic state distillation, which may offer significantadvantages over their qubit counterparts, and hence bring thedevelopment of a scalable quantum computer closer to our grasp.